A fiber-optic cable carries information as flashes of laser light trapped inside a glass core thinner than a human hair. The light stays in the glass through total internal reflection — it bounces off the boundary between the core and its cladding millions of times per mile without escaping — and arrives able to be decoded back into data.
Total internal reflection, in one paragraph
Light bends when it crosses between materials of different density. Make the angle shallow enough and it doesn't cross at all — it reflects completely back inward. Fiber's core and cladding are two glasses tuned so light launched down the core always hits the wall at that shallow angle. The result is a light pipe: photons enter one end, ricochet down miles of glass, and exit the other.
Lasers, wavelengths, and capacity
A transmitter flashes an infrared laser billions of times per second; a receiver reads the flashes. Modern systems multiply capacity by sending many wavelengths (“colors”) of infrared light down the same strand simultaneously — dense wavelength-division multiplexing — which is how one fiber pair carries terabits.
Single-mode fiber (a ~9-micron core) carries one tight beam for tens of miles and is what outside-plant networks use. Multimode (50/62.5-micron core) is cheaper to terminate but limited to short runs inside buildings.
Why splicing is a craft
Two fibers are joined by melting them together in a fusion splicer — an arc welder for glass — aligned to fractions of a micron. A good splice loses almost no light (0.05 dB or less); a bad one dims the signal like a kinked hose. Splicers prove their work with an OTDR, an instrument that shoots light down the strand and maps every splice, bend, and break by its reflection. It's one of the best-paid crafts in fiber construction.